This invention pertains to crosslinked organic-inorganic hybrid membranes. More specifically, this invention pertains to crosslinked poly(ethylene oxide)-cellulose acetate-silsesquioxane (PEO-CA-Si) organic-inorganic hybrid membranes and methods of making these membranes. This invention also pertains to the application of these membranes for gas separations such as CO2/N2 separation.
CO2 is an impurity that must be removed from mixtures with light gases such as CH4, N2 and H2, and the scale of these separations is enormous. See Kohl, et al., GAS PURIFICATION, Fifth Ed., Gulf Publishing, Houston, Tex., 1997. Membrane-based separation of CO2 from gas streams is an important unit operation. Separations of CO2 with membranes include natural gas purification, CO2 capture from flue gas (primarily in mixtures with N2), and metabolic CO2 removal from space life-supporting systems (extravehicular mobility unit (EMU), space shuttle or space station), and CO2 removal from H2.
General public awareness concerning the atmospheric greenhouse or “greenhouse warming” effect attributed to CO2 has created the need to devise environmentally friendly and energy efficient technologies for the removal of CO2 from industrial waste gas streams. Flue gas from fossil fuel power generation (primarily in mixtures with N2) is the largest single contributor to CO2 emissions. Therefore, CO2 recovery from flue gas is becoming more important due to global warming. Generally, flue gas has a large volume and a relatively low concentration in CO2 (typically 10-20 mol-%). Membrane-based separation of CO2 from flue gas holds great promise due to its low energy consumption, low cost, easy operation, and low maintenance. A membrane system with a high processing capacity and a reasonably high selectivity for CO2/N2 is required in order to compete with other separation techniques such as physical or chemical absorption, low temperature distillation and pressure swing adsorption.
Separation of CO2 from flue gas (mainly N2) with commonly used polymeric membranes (e.g. cellulose acetate and fluorine-containing polyimides) or inorganic membranes (e.g. zeolite, sol-gel silica or carbon molecular sieve) is achieved by differences in diffusion rates and/or adsorption strengths of mixture components in the polymer matrix or the inorganic membrane pores, and selectivity is usually rather low, e.g. approximately 20 for CO2/N2 at about 50° C. for gas mixtures. See Baker, IND. ENG. CHEM. RES., 41: 1393 (2002); Tsai, et al., J. MEMBR. SCI., 169: 255 (2000). On the other hand, facilitated transports of CO2 in ion-exchange or immobilized liquid membranes have been intensively investigated because they have high selectivity due to the chemical interaction between CO2 and carrier molecules. See Baltus, et al., SEP. SCI. TECH., 40: 525 (2005). For example, a number of recently developed immobilized liquid membranes exhibited high (>1000) CO2/N2 selectivity due to facilitated CO2 transport mechanism. However, there are still no practical applications for this type of membranes mainly because their CO2 permeation rate is rather low especially at moderate levels of relative humidity (<40%), and also the durability and retention of the liquid in real process conditions are poor. See Kovvali, et al., IND. ENG. CHEM. RES., 40: 2502 (2001); Kovvali, et al., IND. ENG. CHEM. RES., 41: 2287 (2002).
Polymer blends have been the focus of extensive gas membrane separation research since 1980's. For example, poly(ethylene glycol) (PEG) and cellulose acetate or cellulose nitrate blend membranes have been investigated for CO2/N2 separation. It has been reported that PEG can dissolve substantial amounts of sour gases such as CO2, and the diffusivity of large penetrants such as CO2 and CH4 in PEG may be high, considering its flexible main chain. A miscible blend membrane containing 10 wt-% of PEG with molecular weight of 20,000 showed both higher CO2/N2 selectivity and CO2 permeability than those of the cellulose acetate membrane. See Li, et al., J. APPL. POLYM. SCI., 58: 1455 (1995). These blend polymers containing PEG, however, still have issues of durability and retention of the liquid PEG for real industrial applications.
To solve the longer term stability problem of PEG (or poly(ethylene oxide), PEO)-based polymer membranes, various techniques have been reported to crosslink PEG or PEO. For instance, radiation or radical crosslinking of PEG or PEO has been reported. Crosslinking by reactions of end groups such as hydroxyl or vinyl groups has been studied. See Lin et al., J. MOL. STR., 739: 57 (2005); Lin, et al., MACROMOLECULES, 38: 8394 (2005); Lin, et al., MACROMOLECULES, 38: 8381 (2005). All of these crosslinked PEG- or PEO-based membranes are organic polymeric membranes without the presence of inorganic segments, therefore, issues related to chemical resistance, thermal stability, and pressure stability (e.g. plasticization or swelling of membrane) may still exist. In addition, this type of crosslinked organic polymeric membranes has never been fabricated into asymmetric hollow fiber or flat sheet membranes possibly because they cannot be easily integrated into current polymer membrane manufacturing process using phase-inversion technique.